1. Field of the Invention
The invention generally relates to gas lasers and, in particular, relates to methods and apparatus for replenishing gas mixtures within an excimer gas laser.
2. Description of Related Art
Many gas lasers, particularly excimer lasers, maintain a mixture of two or more gasses in a discharge chamber for use in generating a laser beam. A typical excimer laser for example may include a gas mixture composed of fluorine, krypton, and neon. The efficiency of the laser depends, in part, on the particular composition of the gas mixture. A deviation from an optimal composition may decrease the efficiency of the laser, thereby resulting in an output beam of less power. Moreover, a non-optimal gas composition may affect the ability of the laser to maintain a desired output frequency or to maintain a desired pulse rate. A substantial deviation in the gas mixture from an optimal mixture may also affect the durability and reliability of the laser, including causing an increase in corrosion or wear within the laser itself.
The composition of the gas mixture may change as a function of time depending upon several factors. In particular, for fluorine/krypton excimer lasers, the amount of fluorine tends to be depleted while the excimer laser is operated. Fluorine, being a highly reactive halogen, tends to react with materials within the excimer laser by an amount sufficient to lower the amount of fluorine relative to krypton.
For example, a typical fluorine/krypton excimer laser may include a gas mixture composed of 0.1 percent fluorine, 1.0 percent krypton, and 98.9 percent neon. During operation of the excimer laser, fluorine becomes depleted thereby changing the relative compositions described above. The krypton and neon components, being substantially non-reactive noble gasses, are not as significantly depleted as the fluorine.
On possible way to correct for depletion of the fluorine in the laser mix is to completely replace the laser mix with a fresh fill containing the correct relative concentrations of gasses. Although a complete replacement of the gas mixture is an effective way to compensate for a deviation in the relative composition of the gasses, such is not a particularly cost effective or efficient method for compensating for gas depletion. Indeed, excimer laser pre-mixes have become quite expensive and, particularly for large scale excimer lasers having considerable gas chamber volume, the cost of completely flushing the gas chamber and replacing it with new gas can be substantial.
Accordingly, methods have been proposed for compensating for gas component depletion without requiring a complete replacement of the gas mixture. To this end, some fluorine/krypton excimer lasers are provided with a means for adding fluorine to an existing gas mixture, without requiring complete replacement of the gas mixture. During normal operation only fluorine is depleted. Hence, it would be simplest to add only fluorine. However, fluorine is extremely reactive and dangerous. Accordingly, the fluorine must be diluted, typically by neon. Hence, a fluorine source having fluorine diluted by neon (typically 1.0% fluorine) is provided for replenishing depleted fluorine. The fluorine/neon mixture is pumped into the gas chamber in an attempt to compensate for depleted fluorine. With such a technique, the period between complete replacement of the gas mixture can be extended substantially and the overall cost effectiveness of the excimer laser system is improved. However, by providing a replenishment source which includes a mixture of fluorine and neon, the pressure of the laser gas mix will steadily rise due to the addition of neon. Bleed-down is required when the addition of the fluorine/neon gas mixture increases the overall pressure within the chamber above a desired amount, and a portion of the gas mixture must be released to lower the pressure. When the gas pressure is lowered during bleed-down, some amount of krypton is lost. As the cycles of gas injection and bleed-down proceed, the concentration of krypton drops steadily, impairing the efficiency of the laser.